1. Field of the Invention
The present invention relates to a capacitive pressure sensor and, more particularly, to a high accuracy pressure sensor which enables exchange of a multi-structural sensor chip.
2. Description of Related Art
In recent years, the manufacturing techniques of semiconductor devices have been applied to the production of structural components which are essentially sub-millimeter-sized. This technology is called microelectromechanical systems (MEMS) and is widely used to fabricate light and small actuators, sensors and the like in many technical fields. Capacitive pressure sensors can also be produced by MEMS technology, and one example of them is described in JP2001-201417A.
A conventional capacitive pressure sensor of which sensor chip is produced by MEMS technology generally has a configuration that the sensor chip is mounted on a base adaptor with an adhesive, and the base adaptor has a pressure inlet to conduct a gas pressure to the sensor chip. Here, the pressure sensor chip is composed of a silicon substrate and a glass substrate, and a diaphragm electrode and a fixed electrode are formed on the silicon substrate and the glass substrate, respectively. These two electrodes face each other in a sealed chamber which is formed by bonding the silicon substrate and the glass substrate. The pressure sensor having such a configuration is however disadvantageous because the base adaptor must be also replaced when the sensor chip is needed to be exchanged, and results in a wasteful use. Then, the present inventor investigated pressure sensors in which a sensor chip is mounted on a base adaptor detachably by use of a sealing member such as an O-ring, because it is th ought that this method enables the replacement of the sensor chip independently. This sensor is explained in detail with reference to FIGS. 8 and 9. FIGS. 8 and 9 are a schematic sectional view and a perspective view of a conventional capacitive pressure sensor, respectively.
Here, a pressure sensor chip 1 is composed of three substrates, that is, a glass substrate 10 with a fixed electrode 13, a silicon substrate 11 with a diaphragm electrode 15 and a glass substrate 12 with a pressure inlet 18. A multi-structure is formed by bonding those three substrates. The fixed electrode 13 and the diaphragm electrode 15 are connected to an electric circuit (not illustrated) through lead wires 14 and 14′, respectively.
A sealed chamber 16 is formed between a glass substrate 10 and a silicon substrate 11, and the inside of it is kept at low pressure with the aid of a non-evaporable getter 17 which absorbs residual gas inside the sealed chamber 16. The thickness of the diaphragm 15 of the silicon substrate 11 is usually several microns to dozens of microns. The thickness and the size of this diaphragm are determined according to a pressure range to be measured. The diaphragm deflects in response to the pressure difference between both sides of the diaphragm, and the electrostatic capacitance between the fixed electrode 13 and the diaphragm electrode 15 varies as a function of the degree of the diaphragm deflection. Thus, the pressure of the space to be measured 4 can be obtained from the relation between the electrostatic capacitance and the pressure.
As shown in the drawings, the sensor chip 1 is placed on the base adaptor 3 with the intervention of the O-ring 30, and a press plate 2 fixes the sensor chip 1 with plural screws and the like (not illustrated). The pressure sensor in this configuration is installed to a vacuum chamber and the like, and then is used to measure the actual pressure inside the vacuum chamber after zero point adjustment is carried out by adjusting the trimmer potentiometer of a pressure display unit.
Thus, when the sensor chip 1 is damaged or deteriorated, the sensor chip 1 can be exchanged independently, while the base adaptor 3 and other parts are allowed to be reusable. Consequently, it makes possible to reduce the maintenance cost of the pressure sensor.